1. Field of the Invention
The present invention relates generally to auditory prostheses, and more particularly to synchronization in a bilateral auditory prosthesis system.
2. Related Art
Auditory prostheses include any acoustic or electrical auditory prostheses, such as hearing aids, middle ear implants, cochlear implants, brain stem implants, auditory mid-brain implants and other devices that provide electrical and/or acoustic stimulation to a recipient to assist with hearing. Such prostheses receive or generate an electrical signal corresponding to sound. The electrical signal is typically obtained from a microphone that receives the sound and generates a corresponding electrical signal. For example, a conventional cochlear implant includes an external part containing a microphone, sound processor and a headpiece coil; and an implanted part that contains an implant coil, and a stimulator device coupled to an electrode array.
Sound is received at the microphone, which generates an electrical signal that is delivered to the sound processor as an input. The sound processor processes the input signal and generates control signals, according to pre-defined sound processing strategy, for controlling the stimulation of the electrode array of the stimulator device. The control signals are transferred over a transcutaneous link by the headpiece coil via the implant coil to the stimulator device, which sends corresponding stimuli to appropriate electrodes of the electrode array that stimulate the recipient's auditory nerve to cause a perception of hearing.
Bilateral auditory systems include an auditory prosthesis fitted to both the right ear and left ear of a recipient. Each device in a bilateral system may operate independently of the other, or may communicate by either wireless or via a wired connection in delivering joint hearing assistance to the recipient.
One advantage of bilateral systems is the delivery of sound localization cues to a recipient. These cues include the difference in sound arrival time and/or the difference in sound intensity between the two ears.
There are challenges in delivering binaural cues to a recipient via bilateral auditory prostheses. In particular, processing gain varies from prosthesis to prosthesis. For example, louder sounds are suppressed more by automatic gain control (AGC) circuits in each prosthesis than softer sounds. As a result, a loud sound originating, for example, from the left side of a recipient is suppressed more by the left prosthesis than the right prosthesis, where due to head shadow it is received as a softer sound. As a result, the right AGC circuit may not suppress the louder sound at all or suppress it to a lesser extent. This may result in incorrect binaural cues being delivered (i.e. a delayed sound that has a louder or equivalent volume delivered to the right ear than the direct softer sound delivered to the left ear). This tends to adversely affect spatialisation capability, and may ultimately result in creating the impression that all sound is coming from the front.
In addressing these challenges, one approach is to synchronize the two prostheses in a bilateral system, such that the gain of certain events, such as electrical stimulation to auditory nerve fibers enacted by one prosthesis is controlled with respect to the gain in the other prosthesis.
Gain synchronization may be achieved by sending certain data across a communication channel between the two auditory prostheses in a bilateral system. The communication channel may be a wired connection, but in some prostheses the communication channel is an RF channel, such as a 2.4 GHz bidirectional RF channel.
A difficulty arises in using such a communications channel because presently such RF transceivers consume relatively large amounts of power, and this reduces the lifetime of the battery of an auditory prosthesis. Energy consumption may be reduced by operating the transceiver only intermittently or with a lower duty cycle. For example, packetized synchronization data may be sent for short time intervals, repeating every 64 milliseconds. However, the gap between successive synchronization data packets leads to a delay that can be unacceptably slow and incapable of delivering the correct binaural cues.